Google today made a change to their structured data reference page for articles (http://bit.ly/2Ma5CVv). Previously they said:

The only properties supported for non-AMP structured data are the headline and image properties (and related sub-properties such as image.url).

This has been modified to read (emphasis added):

The only properties supported for non-AMP structured data are the headline and image properties (and related sub-properties such as image.url), as well as the dateModified and datePublished properties.

They go on to stipulate, in the markup table for articles (and repeat this language for dateModified:

Add the datePublished property if you want to provide more accurate date information to Google. The Structured Data Testing Tool doesn't show a warning for this property, as it's only recommended if you decide that it's applicable to your site.